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Staali L, Colin DA. Bi-component HlgC/HlgB and HlgA/HlgB γ-hemolysins from S. aureus: Modulation of Ca 2+ channels activity through a differential mechanism. Toxicon 2021; 201:74-85. [PMID: 34411591 DOI: 10.1016/j.toxicon.2021.08.007] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/03/2021] [Revised: 07/21/2021] [Accepted: 08/10/2021] [Indexed: 10/20/2022]
Abstract
Staphylococcal bi-component leukotoxins known as *pore-forming toxins* induce upon a specific binding to membrane receptors, two independent cellular events in human neutrophils. First, they provoke the opening of pre-existing specific ionic channels including Ca2+ channels. Then, they form membrane pores specific to monovalent cations leading to immune cells death. Among these leukotoxins, HlgC/HlgB and HlgA/HlgB γ-hemolysins do act in synergy to induce the opening of different types of Ca2+ channels in the absence as in the presence of extracellular Ca2+. Here, we investigate the mechanism underlying the modulation of Ca2+-independent Ca2+ channels in response to both active leukotoxins in human neutrophils. In the absence of extracellular Ca2+, the Mn2+ has been used as a Ca2+ surrogate to determine the activity of Ca2+-independent Ca2+ channels. Our findings provide new insights about different mechanisms involved in the staphylococcal γ-hemolysins activity to regulate three different types of Ca2+-independent Ca2+ channels. We conclude that (i) HlgC/HlgB stimulates the opening of La3+-sensitive Ca2+ channels, through a cholera toxin-sensitive G protein, (ii) HlgA/HlgB stimulates the opening of Ca2+ channels not sensitive to La3+, through a G protein-independent process, and (iii) unlike HlgA/HlgB, HlgC/HlgB toxins prevent the opening of a new type of Ca2+ channels by phosphorylation/de-phosphorylation-dependent mechanisms.
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Affiliation(s)
- Leila Staali
- Bacteriology Institute of Medical Faculty, Louis Pasteur University, 3 rue Koeberlé, F-67000, Strasbourg, France.
| | - Didier A Colin
- Bacteriology Institute of Medical Faculty, Louis Pasteur University, 3 rue Koeberlé, F-67000, Strasbourg, France
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2
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Thompson JL, Shuttleworth TJ. Anchoring protein AKAP79-mediated PKA phosphorylation of STIM1 determines selective activation of the ARC channel, a store-independent Orai channel. J Physiol 2015; 593:559-72. [PMID: 25504574 PMCID: PMC4324705 DOI: 10.1113/jphysiol.2014.284182] [Citation(s) in RCA: 23] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/09/2014] [Accepted: 12/02/2014] [Indexed: 02/02/2023] Open
Abstract
KEY POINTS Although both the calcium store-dependent CRAC channels and the store-independent ARC channels are regulated by the protein STIM1, CRAC channels are regulated by STIM1 in the endoplasmic reticulum, whilst ARC channels are regulated by the STIM1 constitutively resident in the plasma membrane. We now demonstrate that activation of the ARC channels, but not CRAC channels, is uniquely dependent on phosphorylation of a single residue (T389) in the extensive cytosolic domain of STIM1 by protein kinase A. We further demonstrate that the phosphorylation of the T389 residue by protein kinase A is mediated by the association of plasma membrane STIM1 with the scaffolding protein AKAP79. Together, these findings indicate that the phosphorylation status of this single residue in STIM1 represents a key molecular determinant of the relative activities of these two co-existing Ca(2+) entry channels that are known to play critical, but distinct, roles in modulating a variety of physiologically relevant activities. ABSTRACT The low-conductance, highly calcium-selective channels encoded by the Orai family of proteins represent a major pathway for the agonist-induced entry of calcium associated with the generation and modulation of the key intracellular calcium signals that initiate and control a wide variety of physiologically important processes in cells. There are two distinct members of this channel family that co-exist endogenously in many cell types: the store-operated Ca(2+) release-activated CRAC channels and the store-independent arachidonic acid-regulated ARC channels. Although the activities of both channels are regulated by the stromal-interacting molecule-1 (STIM1) protein, two distinct pools of this protein are responsible, with the major pool of STIM1 in the endoplasmic reticulum membrane regulating CRAC channel activity, whilst the minor pool of plasma membrane STIM1 regulates ARC channel activity. We now show that a critical feature in determining this selective activation of the two channels is the phosphorylation status of a single threonine residue (T389) within the extensive (∼450 residue) cytosolic domain of STIM1. Specifically, protein kinase A (PKA)-mediated phosphorylation of T389 of STIM1 is necessary for effective activation of the ARC channels, whilst phosphorylation of the same residue actually inhibits the ability of STIM1 to activate the CRAC channels. We further demonstrate that the PKA-mediated phosphorylation of T389 occurs at the plasma membrane via the involvement of the anchoring protein AKAP79, which is constitutively associated with the pool of STIM1 in the plasma membrane. The novel mechanism we have described provides a means for the cell to precisely regulate the relative activities of these two channels to independently modulate the resulting intracellular calcium signals in a physiologically relevant manner.
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Affiliation(s)
- Jill L Thompson
- Department of Pharmacology and Physiology, University of Rochester Medical CenterRochester, NY, 14642, USA
| | - Trevor J Shuttleworth
- Department of Pharmacology and Physiology, University of Rochester Medical CenterRochester, NY, 14642, USA
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3
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Evans NE, Forth MKL, Simpson AK, Mason MJ. Inhibition by calyculin A and okadaic acid of the Ca(2+) release-activated Ca(2+) entry pathway in rat basophilic leukemia cells: evidence for regulation by type 1/2A serine/threonine phosphatase activity. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2005; 1718:32-43. [PMID: 16297373 DOI: 10.1016/j.bbamem.2005.10.005] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/04/2005] [Revised: 10/10/2005] [Accepted: 10/12/2005] [Indexed: 11/18/2022]
Abstract
Using a combination of fluorescence measurements of intracellular Ca(2+) ion concentration ([Ca(2+)](i)) and membrane potential we have investigated the sensitivity to serine/threonine phosphatase inhibition of Ca(2+) entry stimulated by activation of the Ca(2+) release-activated Ca(2+) (CRAC) entry pathway in rat basophilic leukemia cells. In both suspension and adherent cells, addition of the type 1/2A phosphatase inhibitor calyculin A, during activation of CRAC uptake, resulted in a fall in [Ca(2+)](i) to near preactivation levels. Pre-treatment with calyculin A abolished the component of the Ca(2+) rise associated with activation of CRAC uptake and inhibited Mn(2+) entry, consistent with a requirement of phosphatase activity for activation of the pathway. Depletion of intracellular Ca(2+) stores is accompanied by a large depolarisation which is absolutely dependent upon Ca(2+) entry via the CRAC uptake pathway. Application of calyculin A or okadaic acid, a structurally unrelated phosphatase antagonist inhibits this depolarisation. Taken in concert, these data demonstrate a marked sensitivity of the CRAC entry pathway to inhibition by calyculin A and okadaic acid.
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Affiliation(s)
- Nicholas E Evans
- Department of Physiology, University of Cambridge, Downing Street, Cambridge CB2 3EG, UK
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4
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Lo KJ, Luk HN, Chin TY, Chueh SH. Store depletion-induced calcium influx in rat cerebellar astrocytes. Br J Pharmacol 2002; 135:1383-92. [PMID: 11906951 PMCID: PMC1573259 DOI: 10.1038/sj.bjp.0704594] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022] Open
Abstract
1. In rat cerebellar astrocytes, intracellular Ca(2+) store depletion by receptor agonists or sarco(endo)plasmic reticulum Ca(2+) ATPase inhibitors induced a transient increase in the intracellular Ca(2+) concentration ([Ca(2+)](i)) in the absence of extracellular Ca(2+) and a sustained increase in its presence. 2. After 10 min treatment with thapsigargin, the [Ca(2+)](i) was unaffected by removal of thapsigargin, but fell rapidly to the basal level when extracellular Ca(2+) was removed, suggesting the involvement of capacitative Ca(2+) entry (CCE); this effect was not seen until cells had been exposed to thapsigargin for at least 2 min. 3. Using the whole cell voltage clamp technique, a 60-100 pA inward current was activated by store depletion, the reversal potential ranging from -5 to 0 mV. 4. When extracellular Na(+) was isotonically replaced by Tris, the thapsigargin-induced [Ca(2+)](i) increase was enhanced, while the inward current was reduced, indicating that store-operated Ca(2+) channels were permeable to Na(+); however, they were not permeable to Sr(2+) or Ba(2+). 5. Thapsigargin-induced CCE remained the same in the presence of nifedipine, La(3+) or Cd(2+), while it was inhibited in the presence of SK&F96365. 6. In cerebellar astrocytes, inhibition of protein serine/threonine phosphorylation promoted CCE. 7. In conclusion, in rat cerebellar astrocytes, store depletion activated a CCE via channels which were permeable to Ca(2+) and Na(+) and regulated by phosphorylation.
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Affiliation(s)
- Kuo-Jung Lo
- Department of Biochemistry, National Defense Medical Center, Taipei, Taiwan, Republic of China
| | - Hsiang-Ning Luk
- Department of Anesthesiology, Chang-Gung Memorial Hospital, Taipei, Taiwan, Republic of China
| | - Ting-Yu Chin
- Department of Biochemistry, National Defense Medical Center, Taipei, Taiwan, Republic of China
| | - Sheau-Huei Chueh
- Department of Biochemistry, National Defense Medical Center, Taipei, Taiwan, Republic of China
- Author for correspondence:
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5
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Abstract
Sixty years after its initial discovery, the octapeptide hormone angiotensin II (AngII) has proved to play numerous physiological roles that reach far beyond its initial description as a hypertensive factor. In spite of the host of target tissues that have been identified, only two major receptor subtypes, AT1 and AT2, are currently fully identified. The specificity of the effects of AngII relies upon numerous and complex intracellular signaling pathways that often mobilize calcium ions from intracellular stores or from the extracellular medium. Various types of calcium channels (store- or voltage-operated channels) endowed with distinct functional properties play a crucial role in these processes. The activity of these channels can be modulated by AngII in a positive and/or negative fashion, depending on the cell type under observation. This chapter reviews the main characteristics of AngII receptor subtypes and of the various calcium channels as well as the involvement of the multiple signal transduction mechanisms triggered by the hormone in the cell-specific modulation of the activity of these channels.
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Affiliation(s)
- M F Rossier
- Department of Internal Medicine, University Hospital, Geneva, Switzerland
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6
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Condrescu M, Hantash BM, Fang Y, Reeves JP. Mode-specific inhibition of sodium-calcium exchange during protein phosphatase blockade. J Biol Chem 1999; 274:33279-86. [PMID: 10559203 DOI: 10.1074/jbc.274.47.33279] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The effects of the protein phosphatase inhibitors calyculin A and okadaic acid on Na(+)/Ca(2+) exchange activity were examined in transfected Chinese hamster ovary cells expressing the bovine cardiac Na(+)/Ca(2+) exchanger. Incubating the cells for 5-10 min with 100 nM calyculin A reduced exchange-mediated (45)Ca(2+) uptake or Ba(2+) influx by 50-75%. Half-maximal inhibition of (45)Ca(2+) uptake was observed at 15 nM calyculin A. The nonselective protein kinase inhibitors K252a and staurosporine provided partial protection against the effects of calyculin A. Okadaic acid, another protein phosphatase inhibitor, nearly completely blocked exchange-mediated Ba(2+) influx. Chinese hamster ovary cells expressing a mutant exchanger in which 420 out of 520 amino acid residues were deleted from the central hydrophilic domain of the exchanger remained sensitive to the inhibitory effects of calyculin A and okadaic acid. Surprisingly, Na(o)(+)-dependent Ca(2+) efflux appeared to be only modestly inhibited, if at all, by calyculin A or okadaic acid. We conclude that protein hyperphosphorylation during protein phosphatase blockade selectively inhibits the Ca(2+) influx mode of Na(+)/Ca(2+) exchange, probably by an indirect mechanism that does not involve phosphorylation of the exchanger itself.
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Affiliation(s)
- M Condrescu
- Department of Pharmacology, University of Medicine and Dentistry of New Jersey, The New Jersey Medical School, Newark, New Jersey 07103, USA
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7
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Chanson M, Mollard P, Meda P, Suter S, Jongsma HJ. Modulation of pancreatic acinar cell to cell coupling during ACh-evoked changes in cytosolic Ca2+. J Biol Chem 1999; 274:282-7. [PMID: 9867842 DOI: 10.1074/jbc.274.1.282] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The temporal changes in cytosolic free Ca2+ ([Ca2+]i), Ca2+-dependent membrane currents (Im), and gap junctional current (Ij) elicited by acetylcholine (ACh) were measured in rat pancreatic acinar cells using digital imaging and dual perforated patch-clamp recording. ACh (50 nM-5 microM) increased [Ca2+]i and evoked Im currents without altering Ij in 19 of 37 acinar cell pairs. Although [Ca2+]i rose asynchronously in cells comprising a cluster, the delay of the [Ca2+]i responses decreased with increasing ACh concentrations. Perfusion of inositol 1,4,5-trisphosphate (IP3) into one cell of a cluster resulted in [Ca2+]i responses in neighboring cells that were not necessarily in direct contact with the stimulated one. This suggests that extensive coupling between acinar cells provides a pathway for cell-to-cell diffusion of Ca2+-releasing signals. Strikingly, maximal (1-5 microM) ACh concentrations reduced Ij by 69 +/- 15% (n = 9) in 25% of the cell pairs subjected to dual patch-clamping. This decrease occurred shortly after the Im peak and was prevented by incubating acinar cells in a Ca2+-free medium, suggesting that uncoupling was subsequent to the initiation of the Ca2+-mobilizing responses. Depletion of Ca2+-sequestering stores by thapsigargin resulted in a reduction of intercellular communication similar to that observed with ACh. In addition, ACh-induced uncoupling was prevented by blocking nitric oxide production with L-nitro-arginine and restored by exposing acinar cells to dibutyryl cGMP. The results suggest that ACh-induced uncoupling and capacitative Ca2+ entry are regulated concurrently. Closure of gap junction channels may occur to functionally isolate nearby cells differing in their intrinsic sensitivity to ACh and thereby to allow for sustained activity of groups of secreting cells.
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Affiliation(s)
- M Chanson
- Department of Medical Physiology and Sport Medicine, Utrecht University, 3508TA Utrecht, The Netherlands.
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8
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Wong K, Li XB. Protein phosphatase inhibitors exert specific and nonspecific effects on calcium influx in thapsigargin-treated human neutrophils. Inflammation 1998; 22:631-42. [PMID: 9824776 DOI: 10.1023/a:1022318631686] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Abstract
C2-ceramide but not inhibitors of phosphatase types 1 and 2A (okadaic acid, calyculin A, tautomycin) blocked store-regulated Ca2+ entry induced in human neutrophils by thapsigargin. This contrasts with previous results showing that both types of compounds inhibit Ca2+ influx in fmet-leu-phe-treated cells. In present studies, phosphatase inhibitors increased the rate of secondary Ca2+ influx in a temperature-dependent manner. Their mechanism of action appeared to be independent of phosphatase inhibition since the inactive congeners, norokadaone and tetraacetyl okadaic acid, also potentiated Ca2+ influx at similar concentrations. When Ca2+ stores were predischarged by thapsigargin, okadaic acid but not norokadaone acted synergistically with fMLP to inhibit subsequent Ca2+ entry. Results suggest that blockade of Ca2+ influx in neutrophils is mediated by a phosphorylation reaction that is prolonged by phosphatase inhibitors. The requisite phosphorylation occurs in fMLP-activated cells but may be absent in cells incubated with thapsigargin.
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Affiliation(s)
- K Wong
- Department of Pharmacology and Therapeutics, University of Calgary, Alberta, Canada
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9
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Paulussen JJ, Fischer MJ, Roozendaal RL, van der Heijden VC, van Dijken P, de Mol NJ, Janssen LH. Effects of oxatomide and derivatives on high affinity IgE receptor-activated signal transduction pathways in rat basophilic leukemia cells: role of protein tyrosine hyperphosphorylation and inhibition of extracellular calcium influx. Biochem Pharmacol 1998; 56:693-701. [PMID: 9751073 DOI: 10.1016/s0006-2952(98)00038-0] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
The antiallergic drug oxatomide and analogs inhibit mediator release from a rat basophilic leukemia (RBL-2H3) cell line, which is frequently used as a mast cell model. By investigating a series of derivatives of oxatomide with different inhibiting activities on exocytosis, we aimed to evaluate the role of their effects on the early steps of the signal transduction cascade in the inhibition of exocytosis. The active compounds induced hyperphosphorylation of tyrosine residues both in stimulated as well as in resting cells. Furthermore, some elevation of the inositol 1,4,5-trisphosphate (IP3) formation upon antigen activation was observed for the active derivatives. Ca2+ fluxes were also studied. The inhibition of the antigen-induced 45Ca2+ influx correlated with the effects of the drugs on exocytosis. Furthermore, the inhibitory activity on antigen- and thapsigargin-mediated exocytosis correlated well. Adherence of the cells to fibronectin, stimulating cellular integrin receptors, was synergistic to antigen activation of the RBL cells. However, oxatomide did lack any effect on integrin-mediated processes, as the IC50 value for exocytosis was identical for fibronectin-adhered cells and standard cultured cells. We conclude that oxatomide and its analogs inhibit exocytosis, mainly by inhibiting Ca2+ influx over store-operated Ca2+ (SOC) channels. The drugs have a direct effect on the store-operated Ca2+ channels or affect the direct regulation of these channels.
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Affiliation(s)
- J J Paulussen
- Department of Medicinal Chemistry, Utrecht Institute for Pharmaceutical Sciences, Faculty of Pharmacy, Utrecht University, The Netherlands
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10
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Fischer MJ, Paulussen JJ, Roozendaal R, Tiemessen RC, de Mol NJ, Janssen LH. Relation between effects of a set of anti-allergic drugs on calcium pathways and membrane structure in Fc epsilon RI activated signal transduction. Inflamm Res 1996; 45:564-73. [PMID: 8951508 DOI: 10.1007/bf02342228] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/03/2023] Open
Abstract
The antigen induced stimulation of mast cells by aggregation of Fc epsilon RI receptors activates a signal transduction cascade leading to release of mediators of inflammation like histamine, arachidonic acid metabolites and cytokines. In this study we investigated a series of structurally related anti-allergic drugs, containing a common lipophilic diphenylmethyl piperazinyl tail and head groups that differ in lipophilicity. Effects of these drugs on various steps of the signal transduction cascade was investigated to gain insight into the mechanism of action of these drugs. It appeared that addition of the drugs to resting cells induced changes in the tyrosine phosphorylation of cellular proteins. The most active anti-allergics in inhibiting exocytosis, AL3264 and oxatomide, also induced the largest changes in phosphorylation. The effects of the drugs on tyrosine phosphorylation after cell activation was complex. Additionally, Ca2+ fluxes were investigated. Ca2+ efflux from the cells was negligibly influenced by the active drugs. However, the drugs inhibited influx from extracellular Ca2+, which was correlated with the effects of the drugs on inhibition of exocytosis and on membrane stabilization induced by the drugs, measured as haemolysis of erythrocytes. It is concluded that inhibition of Ca2+ influx is the major mechanism with which these drugs inhibit exocytosis and that for this effect drug-membrane interactions, possibly affecting the function of membrane embedded proteins, are of importance. Possible mechanisms including drug-membrane interactions, phosphorylation and inhibition of Ca2+ influx are discussed.
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Affiliation(s)
- M J Fischer
- Department of Pharmaceutical Chemistry, Faculty of Pharmacy, Utrecht University, The Netherlands
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11
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Paulussen JJ, Fischer MJ, Kok-Van Esterik JA, Tiemessen RC, De Mol NJ, Janssen LH. Influence of the anti-allergic drug oxatomide on the signal transduction mechanism in a mast cell model. Eur J Pharmacol 1996; 312:121-30. [PMID: 8891587 DOI: 10.1016/0014-2999(96)00453-0] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023]
Abstract
In a mast cell model, oxatomide displays inhibition of mediator release which is not related to its histamine H1 receptor antagonistic activity. From a previous study it appeared that especially early steps in the signal transduction leading to exocytosis were influenced by oxatomide. We now studied effects of oxatomide on those early steps in more detail. The antigen- and thapsigargin-mediated exocytosis in rat basophilic leukemia (RBL-2H3) cells were both inhibited by oxatomide. After aggregation of high affinity receptors for immunoglobulin E (Fc epsilon RI), protein tyrosine phosphorylation is induced. Oxatomide caused remarkable changes in the tyrosine phosphorylation pattern in resting cells. Also after antigen and thapsigargin activation, changes in the tyrosine phosphorylation of cellular proteins are observed. In addition, Ca2+ fluxes were studied by means of the net influx of 45Ca2+ and by measuring intracellular free Ca2+ concentrations ([Ca2+]) with the fluorescent probe fura-2. Oxatomide inhibited the 45Ca2+ influx and the increase in [Ca2+]i upon antigen and thapsigargin activation of the cells. Neither the release of Ca2+ from internal stores nor the efflux of Ca2+ over the plasma membrane seems to be affected. The effect of oxatomide on Ca2+ influx was further characterized by studying Ba2+ influx in the absence of extracellular free Ca2+. We conclude that inhibition of mediator release is mainly caused by inhibition of influx of extracellular Ca2+, via plasma membrane Ca2+ channels that are activated by depletion of intracellular Ca2+ stores. The molecular mechanism with which oxatomide might interfere with these channels is discussed.
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Affiliation(s)
- J J Paulussen
- Department of Pharmaceutical Chemistry, Utrecht Institute for Pharmaceutical Sciences, Faculty of Pharmacy, Utrecht University, Netherlands
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12
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Clementi E, Meldolesi J. Pharmacological and functional properties of voltage-independent Ca2+ channels. Cell Calcium 1996; 19:269-79. [PMID: 8983848 DOI: 10.1016/s0143-4160(96)90068-8] [Citation(s) in RCA: 84] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/03/2023]
Abstract
During the last few years, considerable progress has taken place in our knowledge of the molecular and functional properties of the various voltage-independent Ca2+ channels. In addition to the ionotropic receptor-channels (ROCs), that are not discussed in the present review, these channels include the SMOCs, activated via second messengers or other transducing processes directly triggered by receptor activation; and the SOCCs, activated as a consequence of depletion of the rapidly exchanging Ca2+ stores in the cytoplasm. In parallel, a pharmacological approach to the study of these channels has been developed, based primarily on heterogeneous drugs already known for different biological effects, and subsequently recognized as voltage-independent Ca(2+)-channel blockers. From the systematic analysis of the effects of these drugs new information has emerged about SMOCs and SOCCs function. In addition, pharmacological blockade of these channels appears to have beneficial therapeutic effects in pathological conditions such as tumoral cell growth, inflammation and immunity. At the moment the field is rapidly evolving, with major developments expected in the years ahead.
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Affiliation(s)
- E Clementi
- Department of Pharmacology, University of Reggio Calabria, Catanzaro, Italy.
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13
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Wenzel-Seifert K, Krautwurst D, Musgrave I, Seifert R. Thapsigargin activates univalent- and bivalent-cation entry in human neutrophils by a SK&F I3 96365- and Gd3+-sensitive pathway and is a partial secretagogue: involvement of pertussis-toxin-sensitive G-proteins and protein phosphatases 1/2A and 2B in the signal-transduction pathway. Biochem J 1996; 314 ( Pt 2):679-86. [PMID: 8670085 PMCID: PMC1217100 DOI: 10.1042/bj3140679] [Citation(s) in RCA: 24] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/01/2023]
Abstract
The Ca2+-ATPase inhibitor thapsigargin (TG) activates bivalent-cation early in human neutrophils via depletion of intracellular Ca2+ stores bu little is known about the underlying mechanism and the functional role of TG-induced cation entry. We studied the effects of TG on univalent- and bivalent cation entry, lysozyme release and superoxide-anion (O2-) formation in human neutrophils. TG, like the chemotactic peptide, N-formyl-L-methionyl-L-leucyl-L-phenylalanine (fMLP), stimulated entry of Ca2+, Mn2+, Ba2+, Sr2+ and Na+ in a 1-{beta-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl}-1H-imidazole hydrochloride (SK&F 96365)- and Gd3+-sensitive manner. The inhibitors of protein phosphates 1/2A, calyculin A and okadaic acid, diminished TG-induced cation influxes, whereas the inhibitors of protein phosphatase 2B, cyclosporin A and FK-506, were potentiators. Pertussis toxin (PTX) partially inhibited the effects of TG on Ca2+ and Mn2+ entry. TG and fMLP activated inward currents with a linear current-voltage relationship and a reversal potential at about 0 mV. TG activated lysozyme release and potentiated fMLP-induced O2- formation. TG-induced lysozyme release was inhibited by SK&F 96365, PTX and the removal of extracellular Ca2+ or Na+. Our data show that TG activates a non-selective and SK&F 96365- and Gd3+-sensitive cation entry pathway and is a partial secretagogue. TG-stimulated cation entry involves PTX-sensitive G-proteins and protein phosphatases, with protein phosphatases 1/2A and 2B playing opposite roles.
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Affiliation(s)
- K Wenzel-Seifert
- Institut für Pharmakologie, Universitätsklinikum Benjamin Franklin, Freie Universität Berlin, Federal Republic of Germany
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14
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Tojyo Y, Tanimura A, Matsumoto Y. Suppression of capacitative Ca2+ entry by serine/threonine phosphatase inhibitors in rat parotid acinar cells. JAPANESE JOURNAL OF PHARMACOLOGY 1995; 69:381-9. [PMID: 8786642 DOI: 10.1254/jjp.69.381] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/02/2023]
Abstract
The effects of three serine/threonine protein phosphatase inhibitors, calyculin-A, tautomycin and okadaic acid, on the Ca2+ entry across the plasma membrane was studied in Fura-2-loaded rat parotid acinar cells. These protein phosphatase inhibitors did not affect the peak elevation of cytosolic free Ca2+ concentration ([Ca2+]i) just after stimulation with the muscarinic agonist carbachol (CCh), but they suppressed the sustained increase in [Ca2+]i. In the absence of extracellular Ca2+, CCh produced a transient increase in [Ca2+]i due to Ca2+ release from intracellular Ca2+ stores, and this increase in [Ca2+]i was unaffected by the phosphatase inhibitors. When Ca2+ was added to the external medium after the transient [Ca2+]i response, the increase in [Ca2+]i in the cells treated with the phosphatase inhibitors was significantly smaller than that in the control cells, indicating that the Ca2+ entry was reduced. Similar suppression of Ca2+ entry by the phosphatase inhibitors was observed when intracellular Ca2+ stores were previously depleted by the microsomal Ca(2+)-ATPase inhibitor thapsigargin (TG). In addition, the phosphatase inhibitors reduced the Mn2+ (Ca2+ surrogate) influx following the addition of CCh or TG. The enhancement of Ca2+ entry by the protein kinase inhibitor staurosporine was significantly attenuated by the phosphatase inhibitors. These results suggest that the phosphatase inhibitors suppressed the Ca2+ entry mechanism activated by depletion of intracellular Ca2+ stores in rat parotid acinar cells. The capacitative Ca2+ entry may be regulated by protein phosphorylation/dephosphorylation.
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Affiliation(s)
- Y Tojyo
- Department of Dental Pharmacology, Health Sciences University of Hokkaido, Japan
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15
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Daly JW, Lueders J, Padgett WL, Shin Y, Gusovsky F. Maitotoxin-elicited calcium influx in cultured cells. Effect of calcium-channel blockers. Biochem Pharmacol 1995; 50:1187-97. [PMID: 7488233 DOI: 10.1016/0006-2952(95)00257-z] [Citation(s) in RCA: 38] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/25/2023]
Abstract
Maitotoxin elicited a marked influx of 45Ca2+ into NIH 3T3 fibroblast cells. The influx was blocked by imidazoles (econazole, miconazole, SKF 96365, clotrimazole, calmidazolium) with IC50 values from 0.56 to 3 microM. Phenylalkylamines (verapamil, methoxyverapamil) and nitrendipine were less potent, and diltiazem was very weak. Among other calcium blockers, the diphenylbutylpiperidines fluspirilene and penfluridol, the diphenylpropylpiperidine loperamide, and the local anesthetic proadifen were quite active with IC50 values of 2-4 microM. The pattern of inhibition of maitotoxin-elicited calcium influx did not correspond to the ability of the agents to block elevation of calcium that ensues through calcium-release activated calcium (CRAC) channels after activation of phosphoinositide breakdown by ATP in HL-60 cells. The imidazoles did block CRAC channels, but fluspirilene, penfluridol, loperamide and proadifen were ineffective. Loperamide actually appeared to enhance influx of calcium via the activated CRAC channels. The imidazoles, in particular calmidazolium, caused an apparent influx of calcium and caused a stimulation of phosphoinositide breakdown in HL-60 cells.
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Affiliation(s)
- J W Daly
- National Institutes of Health, Bethesda, MD 20892, USA
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16
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Berlin RD, Preston SF. Arachidonic acid mobilization is suppressed during mitosis: role of cytosolic phospholipase A2 activation. Biochem J 1995; 309 ( Pt 1):91-7. [PMID: 7542448 PMCID: PMC1135804 DOI: 10.1042/bj3090091] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/25/2023]
Abstract
In interphase HeLa cells, incubation with histamine or thapsigargin led to the rapid release of arachidonic acid. The release was absolutely dependent on Ca2+, consistent with the activation of an 85 kDa cytosolic phospholipase A2 (cPLA2). In metaphase-arrested HeLa cells, by contrast, the stimulation of arachidonate release by these agents was inhibited by more than 90%. The lack of arachidonic acid release by mitotic cells was at least partly expected, since histamine- or thapsigargin-induced Ca2+ influx and elevations of cytosolic free Ca2+ are known to be strongly inhibited during mitosis [Preston, Sha'afi and Berlin (1991) Cell Regul. 2, 915-925]. Indeed, incubation of interphase cells with the Ca2+ ionophore A23187 alone induced a high level of arachidonate release. However, even A23187 failed to elicit release from mitotic cells. Since the Ca(2+)-dependent release of arachidonate by many cell types is promoted by preincubation with ligands that activate receptors of the tyrosine kinase class, and tumour promoters that lead to the phosphorylation of cPLA2, we determined if the responses of mitotic HeLa cells could be modified by this 'priming' process. We first established that epidermal growth factor and phorbol 12-myristate 13-acetate were effective priming agents in interphase cells: cells preincubated with the hormone or tumour promoter showed a 2-fold stimulation of thapsigargin- or A23187-induced arachidonic acid release. However, none of the priming agents reversed the lack of mitotic cell response. This refractoriness was not caused by destruction of cPLA2 during mitosis: by Western blotting, cPLA2 of interphase and mitotic cells was shown to be present in comparable amounts. Moreover, cPLA2 activities measured in extracts of interphase and mitotic cells were also comparable. Surprisingly, mitotic cPLA2 appeared to be constitutively phosphorylated in non-hormone-treated (control) cells. The results indicate a novel mechanism of regulation by cPLA2 activity in mitotic cells.
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Affiliation(s)
- R D Berlin
- Department of Physiology, University of Connecticut Health Center, Farmington, USA
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17
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Abstract
Spatiotemporal Ca2+ signalling in the cytoplasm is currently understood as an excitation phenomenon by analogy with electrical excitation in the plasma membrane. In many cell types, Ca2+ waves and Ca2+ oscillations are mediated by inositol 1,4,5-trisphosphate (IP3) receptor/Ca2+ channels in the endoplasmic reticulum membrane, with positive feedback between cytosolic Ca2+ and IP3-induced Ca2+ release creating a regenerative process. Remarkable advances have been made in the past year in the analysis of subcellular Ca2+ microdomains using confocal microscopy and of Ca2+ influx pathways that are functionally coupled to IP3-induced Ca2+ release. Ca2+ signals can be conveyed into the nucleus and mitochondria. Ca2+ entry from outside the cell allows repetitive Ca2+ release by providing Ca2+ to refill the endoplasmic reticulum stores, thus giving rise to frequency-encoded Ca2+ signals.
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Affiliation(s)
- S Miyazaki
- Department of Physiology, Tokyo Women's Medical College, Japan
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18
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Wong K, Li XB, Hunchuk N. N-acetylsphingosine (C2-ceramide) inhibited neutrophil superoxide formation and calcium influx. J Biol Chem 1995; 270:3056-62. [PMID: 7852386 DOI: 10.1074/jbc.270.7.3056] [Citation(s) in RCA: 57] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023] Open
Abstract
Ceramide, a product arising from sphingomyelinase activity, has been shown to act as an intracellular second messenger in effecting growth inhibition, cellular differentiation, and apoptosis. In the present study, the relative effects of cell-permeable ceramides, N-acetylsphingosine (C2-ceramide) and N-hexanoylsphingosine (C6-ceramide), on neutrophil responses were measured. When cells were activated with fMet-Leu-Phe, C2-ceramide both potentiated (< 1 microM) and inhibited (> 1 microM) superoxide generation. C2- and C6-ceramide inhibited phorbol ester-induced superoxide release from neutrophils at IC50 values of 5 and 120 microM, respectively. C2-ceramide had no effect on semipurified protein kinase C activity. Neither ceramide affected significantly the general level of phosphorylated proteins in phorbol ester-treated cells. C2-ceramide (1-20 microM) alone did not change cytosolic free Ca2+ levels but inhibited Ca2+ and Mn2+ influx in fMet-Leu-Phe-activated neutrophils. In contrast, sphingosine enhanced Ca2+ entry; thus, ceramide conversion to sphingosine was not significant. Unlike C2-ceramide, C2-dihydroceramide failed to block superoxide generation or Ca2+ influx. Preincubation of cells with 10 nM okadaic acid reversed slightly the effects of C2-ceramide. Calyculin A, tautomycin, and much higher concentrations of okadaic acid inhibited agonist-induced Ca2+ influx. We postulate that C2-ceramide may inhibit neutrophil superoxide release by activation of type 2A protein phosphatases. Results suggest that protein phosphatase type 1 up-regulates Ca2+ entry, whereas type 2A (or a ceramide-activated subtype) forestalls Ca2+ entry by inactivating a calcium influx factor.
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Affiliation(s)
- K Wong
- Department of Pharmacology and Therapeutics, University of Calgary, Alberta, Canada
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19
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Randriamampita C, Tsien RY. Degradation of a calcium influx factor (CIF) can be blocked by phosphatase inhibitors or chelation of Ca2+. J Biol Chem 1995; 270:29-32. [PMID: 7814389 DOI: 10.1074/jbc.270.1.29] [Citation(s) in RCA: 102] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023] Open
Abstract
In many cell types, depletion of Ca2+ stores causes activation of Ca2+ influx by a mechanism whose molecular basis remains unclear. We recently described a new messenger that is released by empty Ca2+ stores and that activates Ca2+ influx in heterologous cells (Randriamampita, C. & Tsien, R. Y. (1993) Nature 364, 809-814). This factor, provisionally named CIF (for Ca2+ influx factor), seems to be a small nonprotein factor possessing a phosphate group. Meanwhile Parekh et al. reported that okadaic acid, an inhibitor of protein phosphatases 1 and 2A, potentiates Ca2+ influx in Xenopus oocytes (Parekh, A. B., Terlau, H. & Stühmer, W. (1993) Nature 364, 814-818). A link between these two observations is presented in this paper. We show that in astrocytoma cells, okadaic acid and cyclosporin A (an inhibitor of calcineurin) both potentiate the Ca2+ elevations due to low doses of CIF, thapsigargin, or carbachol. In lymphocytes, okadaic acid potentiates the Ca2+ elevations due to low doses of phytohemagglutinin and increases the amount of extractable CIF. CIF degradation can be observed in cell-free homogenates of lymphocytes and is prevented by the above phosphatase inhibitors, an effect that can at least partly explain their potentiation of Ca2+ influx. CIF degradation is also prevented by lowering free Ca2+ concentrations, which could be a feedback mechanism to enhance Ca2+ influx when cells are depleted of Ca2+.
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Affiliation(s)
- C Randriamampita
- Laboratoire de Neurobiologie, Ecole Normale Supérieure, Paris, France
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20
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Venant A, Dazy AC, Diogène G, Marano F. Differential effects of maitotoxin on calcium entry and ciliary beating in the rabbit ciliated tracheal epithelium. Biol Cell 1995; 85:197-205. [PMID: 8785521 DOI: 10.1016/0248-4900(96)85281-6] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023]
Abstract
The marine toxin maitotoxin (MTX) induces stimulation of ciliary beating in primary cultures of rabbit tracheal epithelial cells. The response is time- and concentration-dependent. External calcium is an absolute requirement, although at a very low concentration (50 microM for maximal effect). Pretreatment of the cells with MTX induces an early (5 min) and sustained ( > or = 24 h) homologous desensitization. The response to MTX is strongly inhibited by trifluoperazin (an inhibitor of Ca-calmodulin-dependent enzymes) and by chelation of [Ca]i with BAPTA. However, the magnitude and kinetics of [Ca]i rise elicited by MTX do not correlate with those of the ciliary beat frequency (CBF) increase: the CBF increase is transient (with a peak at 5-10 min) while the [Ca]i rise is sustained; the CBF increase occurs at concentrations of MTX which are without an effect on [Ca]i; the CBF increase is not inhibited by 200 microM verapamil, genistein or okadaic acid, which inhibit the MTX-induced [Ca]i rise. The CBF increase is strongly inhibited by antagonists of arachidonic acid metabolism, mepacrine and nordiguaiaretic acid. However, MTX does not stimulate cAMP synthesis. These results suggest that calcium is not the only factor involved in the biological effects of MTX and even suggest that MTX may primarily stimulate phospholipid breakdown in the cell membrane.
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Affiliation(s)
- A Venant
- Laboratoire de Cytophysiologie et Toxicologie Cellulaire, Universitè Paris, France
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[Ca2+]i inhibition of Ca2+ release-activated Ca2+ influx underlies agonist- and thapsigargin-induced [Ca2+]i oscillations in salivary acinar cells. J Biol Chem 1994. [DOI: 10.1016/s0021-9258(18)31726-5] [Citation(s) in RCA: 31] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022] Open
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Davis MA, Smith MW, Chang SH, Trump BF. Characterization of a renal epithelial cell model of apoptosis using okadaic acid and the NRK-52E cell line. Toxicol Pathol 1994; 22:595-605. [PMID: 7732277 DOI: 10.1177/019262339402200604] [Citation(s) in RCA: 30] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023]
Abstract
Apoptotic cell death plays an important role in the pathogenesis of renal tubular epithelial damage and repair following tubular injury. Presently, the cellular factors involved in regulating apoptotic pathways in the kidney are unknown. To address the possibility that protein phosphorylation may regulate apoptosis in kidney cells, okadaic acid (OKA), a specific inhibitor of protein phosphatases 1A and 2A, was tested for its morphologic and biochemical effects on normal rat kidney epithelial cells (NRK-52E) in culture. As revealed by the DNA-specific stain DAPI, nuclei of cells treated with 1.0 microM okadaic acid contained irregular clumps of dense chromatin. Additional morphologic alterations typical of apoptosis were apparent within 2 hr after treatment with 1.0 microM OKA, including marked cellular rounding, cytoplasmic condensation, and cytoplasmic blebs. Ultrastructurally, 1.0 microM OKA caused cytoplasmic bleb formation, cellular fragmentation, condensation of heterochromatin into clumps, and segregation of nucleoli. At this stage, the cytoplasmic fragments and blebs contained many normal mitochondria. The attached, rounded cells also effectively excluded propidium iodide, demonstrating maintenance of membrane integrity despite pronounced morphologic alterations. A 2-fold increase in intracellular free Ca2+ was apparent 90 min after treatment with 1.0 microM okadaic acid. Transverse alternating field electrophoresis revealed the appearance of large DNA fragments of approximately 300-kbp. The appearance of these 300-kbp fragments correlated temporally with the observed elevation in intracellular calcium and the onset of morphologic alterations. However, preloading cells with EGTA-AM, an intracellular calcium chelator, obliterated the calcium elevation and had no effect on OKA-induced morphology, DNA fragmentation, or cell death. Detectable internucleosomal fragmentation occurred much later than the onset of morphologic changes (24-hr treatment time) and did not correlate with elevations in cellular calcium. These studies support the hypothesis that during apoptosis, chromatin condensation reflects chromatin cleavage at nuclease-sensitive sites between hexameric rosettes. These results also suggest that morphologic and nuclear alterations in the pathway of OKA-induced apoptosis occur independent of observed increases in intracellular calcium.
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Affiliation(s)
- M A Davis
- Department of Pathology, University of Maryland School of Medicine, Baltimore 21201, USA
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Wang XL, Akhtar RA, Abdel-Latif AA. Studies on the properties of myo-inositol-1,4,5-trisphosphate 5-phosphatase and myo-inositol monophosphatase in bovine iris sphincter smooth muscle: effects of okadaic acid and protein phosphorylation. BIOCHIMICA ET BIOPHYSICA ACTA 1994; 1222:27-36. [PMID: 8186262 DOI: 10.1016/0167-4889(94)90021-3] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/29/2023]
Abstract
In bovine iris sphincter, myo-inositol 1,4,5-trisphosphate (IP3) 5-phosphatase and myo-inositol 1-phosphate (IP1) monophosphatase are mainly localized in the microsomal and soluble fractions, respectively. Studies on the properties of these enzymes can be summarized as follows. (1) The microsomal IP3 5-phosphatase hydrolyzed IP3 to myo-inositol 1,4-bisphosphate with an apparent Km of 28 microM and Vmax of 32 nmol/min per mg protein. The IP1 monophosphatase in the soluble fraction hydrolyzed IP1 into free inositol with an apparent Km of 89 microM and Vmax of 7 nmol/min per mg protein. (2) IP3 5-phosphatase and IP1 monophosphatase had optimal pH values at 8.0 and 7.0, respectively. (3) Both enzymes required Mg2+ and their highest specific activities were at a cation concentration of 2 mM. (4) Ca2+ (> 0.5 microM) exerted an inhibitory effect on IP3 5-phosphatase activity, and marked inhibition (47%) was observed at a concentration of 10 microM. Higher concentrations of the cation (> 100 microM) were required to inhibit IP1 monophosphatase. (5) IP1 monophosphatase, but not IP3 5-phosphatase, was inhibited by Li+. Li+ had no effect on the contractile response in this smooth muscle. (6) Both enzymes were inhibited by ATP and by the thiol-blocking agent, disulfiram. In addition, thimerosal, a thiol reagent, also inhibited the IP3 5-phosphatase activity. (7) Protein phosphorylation of the microsomal and soluble fractions with PKA or PKC had no effect on the activities of these enzymes. (8) Okadaic acid, a protein phosphatase inhibitor, had no effect on the activity of IP3 5-phosphatase. However, in the intact iris sphincter the toxin significantly reduced the carbachol-induced IP3 production, 1,2-diacylglycerol formation, measured as phosphatidic acid, and caused muscle relaxation.
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Affiliation(s)
- X L Wang
- Department of Biochemistry and Molecular Biology, Medical College of Georgia, Augusta 30912
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Chandra S, Fewtrell C, Millard P, Sandison D, Webb W, Morrison G. Imaging of total intracellular calcium and calcium influx and efflux in individual resting and stimulated tumor mast cells using ion microscopy. J Biol Chem 1994. [DOI: 10.1016/s0021-9258(17)36590-0] [Citation(s) in RCA: 31] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022] Open
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25
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Koike Y, Ozaki Y, Qi R, Satoh K, Kurota K, Yatomi Y, Kume S. Phosphatase inhibitors suppress Ca2+ influx induced by receptor-mediated intracellular Ca2+ store depletion in human platelets. Cell Calcium 1994; 15:381-90. [PMID: 8033196 DOI: 10.1016/0143-4160(94)90013-2] [Citation(s) in RCA: 28] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023]
Abstract
The effects of three phosphatase inhibitors including okadaic acid, calyculin A and tautomycin were evaluated on platelet Ca2+ mobilization. Calyculin A and tautomycin at appropriate concentrations appeared to have a selective inhibitory effect on thrombin-induced Ca2+ influx, but not on [Ca2+]i release from intracellular Ca2+ storage sites. In contrast, pretreatment with okadaic acid at concentrations that effectively lowered Ca2+ influx also suppressed Ca2+ release from intracellular Ca2+ stores. In a system that specifically evaluates the effects of agents on Ca2+ influx induced by the Ca(2+)-depleted state of intracellular Ca2+ storage sites, the three phosphatase inhibitors attenuated Ca2+ influx in a dose dependent manner and showed complete inhibition at appropriate concentrations. These findings suggest that protein phosphorylation/dephosphorylation plays an important role in mediating signals to open Ca2+ channels when Ca2+ depletion in intracellular Ca2+ stores is caused by thrombin. In contrast, Ca2+ influx induced by thapsigargin, a Ca(2+)-ATPase inhibitor, was only partially suppressed by pretreatment with each of the three phosphatase inhibitors. Based on these findings, we suggest that the Ca(2+)-depleted state of intracellular Ca2+ stores by thapsigargin induces the opening of Ca2+ channels via phosphatase inhibitor-insensitive pathways. All the phosphatase inhibitors, at the highest concentrations tested in the present study, only partially inhibited Mn2+ entry induced by thrombin. These findings suggest that there are at least two types of divalent ion channels on platelet plasma membranes and that one of them, that preferentially allows Mn2+ entry, is resistant to the inhibitory effects of phosphatase inhibitors.
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Affiliation(s)
- Y Koike
- Department of Clinical and Laboratory Medicine, Yamanashi Medical College, Japan
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Abstract
By virtue of their biological simplicity and widespread availability, platelets frequently have been used as a model system to study signal transduction. Such studies have revealed that changes in intracellular free calcium concentration are central to platelet functioning. The following article reviews current concepts of platelet structure and function, with particular emphasis on the mechanisms involved in platelet Ca2+ signalling.
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Affiliation(s)
- P Sargeant
- Physiological Laboratory, Cambridge University, U.K
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28
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Affiliation(s)
- J W Putney
- Laboratory of Cellular and Molecular Pharmacology, National Institute of Environmental Health Sciences, National Institutes of Health, Research Triangle Park, North Carolina 27709
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